Question

why pcl5 is unstable and dissosiate to give pcl3 and pcl2 ?

Answer 1

PCl5 has a trigonal pyramidal shape. The bond pair - bond pair repulsion is greater in the two axial P-Cl bonds as compared to the three equatorial P-Cl bonds. Due to this, the axial P-Cl bonds are less stable and are removed readily when PCl5 is heated.
PCl5 ⇔PCl3 + Cl2